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cg++c-preprocessorvariadic
hasen
hasen

Reputation: 166342

How to make a variadic macro (variable number of arguments)

I want to write a macro in C that accepts any number of parameters, not a specific number

example:

#define macro( X )  something_complicated( whatever( X ) )

where X is any number of parameters

I need this because whatever is overloaded and can be called with 2 or 4 parameters.

I tried defining the macro twice, but the second definition overwrote the first one!

The compiler I'm working with is g++ (more specifically, mingw)

Upvotes: 248

Views: 278334

Answers (6)

cmccabe
cmccabe

Reputation: 4340

__VA_ARGS__ is the standard way to do it. Don't use compiler-specific hacks if you don't have to.

C++ is not a superset of C. It is really silly to compile your C code with a C++ compiler.

Upvotes: 53

Alex B
Alex B

Reputation: 84962

C99 way:

#define FOO(...) printf(__VA_ARGS__)

Upvotes: 364

eduffy
eduffy

Reputation: 40252

I don't think that's possible, you could fake it with double parens ... just as long you don't need the arguments individually.

#define macro(ARGS) some_complicated (whatever ARGS)
// ...
macro((a, b, c))
macro((d, e))

Upvotes: 32

Kateridzhe
Kateridzhe

Reputation: 249

• Variable number of arguments is denoted by an ellipsis (...) • The syntax of ISO C requires at least one fixed argument before the ‘...’

For example, you can type:

#define DEBUGMSG ( int, ...)

Upvotes: 1

Symon
Symon

Reputation: 

#define DEBUG

#ifdef DEBUG
  #define PRINT print
#else
  #define PRINT(...) ((void)0) //strip out PRINT instructions from code
#endif 

void print(const char *fmt, ...) {

    va_list args;
    va_start(args, fmt);
    vsprintf(str, fmt, args);
        va_end(args);

        printf("%s\n", str);

}

int main() {
   PRINT("[%s %d, %d] Hello World", "March", 26, 2009);
   return 0;
}

If the compiler does not understand variadic macros, you can also strip out PRINT with either of the following:

#define PRINT //

or

#define PRINT if(0)print

The first comments out the PRINT instructions, the second prevents PRINT instruction because of a NULL if condition. If optimization is set, the compiler should strip out never executed instructions like: if(0) print("hello world"); or ((void)0);

Upvotes: 13

DarenW
DarenW

Reputation: 16926

explained for g++ here, though it is part of C99 so should work for everyone

http://www.delorie.com/gnu/docs/gcc/gcc_44.html

quick example:

#define debug(format, args...) fprintf (stderr, format, args)

Upvotes: 10

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